Turn off editor marks toggle on enter

[erquhart]

- Do you want to request a feature or report a bug?
Improvement

- What is the current behavior?
If strong or emphasis is on in the markdown editor, and you press enter, the toggles remain on, but will turn off when further text is entered.

If you paste instead typing after pressing Enter, the pasted text will take on the toggled styles.

- If the current behavior is a bug, please provide the steps to reproduce.

• Type style text in the markdown editor
• With the style still on, press Enter

- What is the expected behavior?
Style toggles should turn off on Enter, and further typed or pasted content should not be styled.

- Please mention your CMS, node.js, and operating system version.
CMS 0.5.1